Method for Operating a Piezo Injector

ABSTRACT

The present disclosure relates to fuel injectors. The teachings may be embodied in a method for characterizing a hydraulic coupling element. The fuel injector may have a piston to pressurize a hydraulic medium and a pin connecting the piston to a piezoactuator. The method may include applying a charging current to the piezo actuator low enough that the leakage flow prevents a pressure differential and the nozzle needle remains closed; discharging the piezo actuator with a current high enough to release the mechanical connection between the piston and the pin; detecting when the piston impacts on the pin; and characterizing the coupling element based on the time between discharge and impact.

The present invention relates to a method for characterizing a hydraulic coupling element having a piston which places a hydraulic medium under pressure and a pin which connects said piston to a piezo actuator, said coupling element converting the translatory stroke of the piezo actuator into a pressure differential which opens the nozzle needle of a piezo injector.

A new generation of fuel injection systems, in particular common-rail fuel injection systems, operates with directly driven piezo injectors which actuate the nozzle needle via a hydraulic coupling element. In this context the coupling element converts the translatory stroke of the piezo drive into a pressure differential which opens the nozzle needle. The coupling element comprises a piston and a pin which connects said piston to a piezo actuator. The piston is located in a pressure cylinder. As a result of the lengthening of the piezo actuator, pressure is applied to fuel which is located in the pressure cylinder and is under high pressure, as a result of which a nozzle needle opens an injection opening and fuel is thereby injected into a combustion chamber. As a result of discharging of the piezo actuator, it shortens in length, as a result of which the piston of the coupling element moves back and therefore brings about a reduction in pressure of the fuel, which reduction causes the nozzle needle to close the injection opening.

The transmission properties of such a coupling element are, apart from the physical characteristic variables of the fuel, dependent to a high degree on the leakage fluid that flows on the coupling element. A balance of inflowing, outflowing and circulating quantities is established, which balance influences the respective dynamic transmission behavior and therefore the opening and closing of the needle. This balance changes as a function of temperature, viscosity, component tolerance and aging, i.e. the gap cross section. In order to provide a correction of the actuation, it is useful to know the state of this balance.

It is known here to model the temperature influence statistically by means of characteristic diagrams. However, until now it has not been possible to detect the aging and component tolerance of such a hydraulic coupling element.

The present invention is based on the object of making available a method of the type which is reproduced at the beginning and with which particularly precise actuation of the piezo actuator can be achieved.

This object is achieved according to the invention by means of a method of the specified type which has the following steps:

carrying out a test actuation of the piezo actuator with a very low charging current, with the result that the piezo actuator moves so slowly that the leakage flow produced by the coupling element prevents a pressure differential, and the nozzle needle therefore remains closed;

discharging of the piezo actuator by a very high current, with the result that the mechanical connection between the piston and the pin is released;

generating a signal when the piston impacts on the pin;

detecting this signal;

measuring the time from the start of discharging to the impacting of the piston on the pin; and

using the measured time to characterize the coupling element.

According to the invention, the coupling element is subjected to a test actuation. This test actuation takes place in such a way that as a result of a very low charging current the piezo actuator is moved so slowly that the leakage circulation which is produced prevents a pressure differential, and the nozzle needle therefore remains closed. The piezo actuator is then discharged by a high current. The piston of the coupling element cannot follow the rapid movement of the piezo actuator at the same speed, with the result that the mechanical connection between the piston and the pin is released. The piston follows with a damped speed, which is limited by the possibility of equalizing the fluid volume upstream and downstream of the piston. When the piston impacts on the pin of the actuator, the force effect acting on the piezo element causes a signal to be generated, which can be detected as a change in capacitance or voltage or current.

The time from the start of discharging until the impacting of the piston characterizes the thermo-hydraulic and tribological state of the coupling element, and is used to characterize the coupling element.

The measured time period is therefore used as an indication of the state of the coupling element. It is therefore possible to assume, for example, that given a relatively short time period a relatively high degree of wear of the coupling element is present, since a relatively large gap is present between the wall of the pressure cylinder and the piston, and there is therefore a relatively strong circulation around the piston. Conversely, in the case of a relatively long time period, the wear of the piston is relatively low, since the circulation gap is only small.

In a development of the method according to the invention, the measured time is therefore used to monitor the wear of the coupling element.

A further variant of the method according to the invention is distinguished by the fact that the measured time is used to correct the actuation of the piezo injector.

The invention will be explained in detail below using an exemplary embodiment and in conjunction with the drawing, in which:

FIG. 1 shows a schematic illustration of an injector which is driven directly by means of a piezo actuator and a hydraulic coupling element;

FIG. 2 shows the actuation profile for the test actuation of the piezo actuator;

FIG. 3 shows a schematic illustration like FIG. 1, which shows the piezo actuator in the extended state without movement of the nozzle needle; and

FIG. 4 shows an illustration corresponding to FIG. 3, which shows the piezo actuator in the contracted state with a damped resetting movement of the piston.

The injector which is illustrated schematically in FIG. 1 can be, for example, part of an injection system, having a pressure accumulator (rail), of a motor vehicle. The injector has a nozzle needle 7 which opens and closes an injection opening 8.

A spring 9 presses the nozzle needle 7 downward in the figure, in order to close the injection opening 8. Fuel under high pressure is fed via the line 5. If the pressure of the fuel exceeds the pressure applied by the spring 9, the nozzle needle 7 is moved upward in the figure, in order to open the injection opening 8 and to inject a metered quantity of fuel into a combustion chamber. If the fuel pressure drops, the injection opening 8 is closed again by the nozzle needle 7 as a result of the action of the spring 9.

The drive of the injector is provided here via a piezo actuator 1 and a coupling element which converts the translatory stroke of the piezo actuator 1 into a pressure differential which opens the nozzle needle. Here the, the piezo actuator 1 is connected via a pin 2 to a piston 3 which has a loose mechanical coupling to the pin 2. The piston 3 moves in a pressure cylinder 6 in which a spring 4 is arranged. By lengthening the piezo actuator 1, the piston 3 is moved downward in the figure via the pin 2 counter to the force of the spring 4, and in the process places the fuel, flowing by the line 5, under pressure, so that the nozzle needle 7 opens the injection opening 8 and a corresponding quantity of fuel is injected. At 10, a gap 10 which is present between the pressure cylinder 6 and the piston surface is illustrated, through which gap 10 a leakage flow flows past the piston. A corresponding leakage flow flows past the nozzle needle via the gap 11 and into the associated coupling space.

In order to be able to characterize the coupling element which comprises the piston 3, a test actuation of the piezo actuator 1 is carried out. In this context, a very low charging current is applied to the piezo actuator 1, with the result that said piezo actuator 1 moves so slowly that the leakage flow produced by the coupling element prevents a pressure differential, and the nozzle needle 7 therefore remains closed. This state is illustrated in FIG. 3.

Then, the piezo actuator 1 is discharged by means of a very high current, with the result that the mechanical connection between the piston 3 and pin 2 is released. This is attributable to the fact that the piston 3 cannot follow the rapid movement of the piezo actuator 1 at the same speed. This state is shown in FIG. 4. The piston 3 instead follows with a damped speed. When the piston 3 impacts on the pin 2, the force effect on the piezo element causes a signal to be generated, which is detected, for example, as a change in capacitance. The time from the start of discharging up to the impact of the piston 3 on the pin 2 is then measured, and this time is used to characterize the coupling element.

FIG. 2 shows the actuation profile for the test actuation of the piezo actuator. In this context, the current is represented on the ordinate, and the time is represented on the abscissa. A typical charging current for activation of an injector is characterized by 20, said current not being used in the method described here. Instead, the operation is carried out here with the low charging current (shown at 21) for the test pulse. The low charging current is followed here by the high discharging current (represented at 22) which gives rise to a separation between the piston 3 and pin 2.

With the measured time, for example the wear of the coupling element or piston 3 can be detected. If the time is relatively short, the circulation around the piston 3 is relatively large, so that the wear can therefore be classified as high. If the measured time is, on the other hand, long, low circulation, and therefore a low level of wear, can be assumed. 

What is claimed is:
 1. A method for controlling a fuel injector including a hydraulic coupling element with a piston placing a hydraulic medium under pressure and a pin which connects said piston to a piezo actuator, wherein the coupling element converts a translational stroke of the piezo actuator into a pressure differential to open the nozzle needle of a piezo injector, the method comprising: applying a charging current to the piezo actuator, the charging current low enough that the leakage flow produced by the coupling element prevents a pressure differential and the nozzle needle remains closed; discharging the piezo actuator with a current high enough to release the mechanical connection between the piston and the pin; generating a signal when the piston impacts on the pin; detecting the signal; measuring the time from a start of discharging to the detection of the impact of the piston on the pin; characterizing the coupling element based on the measured time; and controlling the actuation of the fuel injector based on the characterization of the coupling element.
 2. The method as claimed in claim 1, wherein the measured time is used to monitor the wear of the coupling element.
 3. (canceled)
 4. A method for testing a fuel injector including a hydraulic coupling element with a piston placing a hydraulic medium under pressure and a pin which connects said piston to a piezo actuator, wherein the coupling element converts a translational stroke of the piezo actuator into a pressure differential to open the nozzle needle of a piezo injector, the method comprising: applying a charging current to the piezo actuator, the charging current low enough that the leakage flow produced by the coupling element prevents a pressure differential and the nozzle needle remains closed; discharging the piezo actuator with a current high enough to release the mechanical connection between the piston and the pin; generating a signal when the piston impacts on the pin; detecting the signal; measuring the time from a start of discharging to the detection of the impact of the piston on the pin; characterizing the coupling element based on the measured time; and the characterization of the coupling element represents mechanical wear of the coupling element.
 5. The method as claimed in claim 4, further comprising controlling the actuation of the fuel injector based on the mechanical wear of the coupling element.
 6. A fuel injector comprising: a piezoactuator driven by a current source; a coupling element driven by the piezoactuator and converting extension of the piezoactuator into a pressure differential; a nozzle needle actuated by the pressure differential; a gap defined between the coupling element and a housing wall, the gap allowing a fuel to flow past the coupling element to the nozzle needle; wherein the current source generates a current low enough that leakage around the coupling element keeps the pressure differential low enough to prevent opening of the nozzle needle; and the current source discharges the piezo actuator with a current high enough to release a mechanical connection between at least a part of the coupling element and the piezoactuator; and a processor measuring the time from a start of discharging to the impact of the piston on the pin, characterizing the coupling element based on the measured time, and operating the fuel injector based on the characterization.
 7. A fuel injector as claimed in claim 6, wherein the coupling element comprises a pin with a loose mechanical connection to a piston.
 8. A fuel injector as claimed in claim 6, wherein the coupling element comprises a pin with a loose mechanical connection to a piston; and the piston moves in a pressure cylinder against a spring arranged in the pressure cylinder; wherein extension of the piezoactuator moves the piston against the force of the spring to pressurize a fuel and the nozzle needle opens in response to the pressurization. 